This invention relates to a wedge type grip for use in material testing machines, which is provided with a pair of wedge-shaped gripping members or jaws for holding a specimen to be tested.
FIG. 3 diagramatically shows a known wedge type grip comprising a grip frame or yoke 10 and a pair of wedge-shaped gripping members or jaws 20A and 20B which will be designated by only the numeral 20 without the suffix A or B when it is not necessary to discriminate between the two members 20A and 20B. The yoke 10 is formed with an inner space 3 which gradually expands from its entrance 1 toward its inner end 2, with a pair of inclined faces 11 formed opposite to each other and symmetrically with respect to an axis X of loading. Each of the jaws 20A and 20B is formed with an inclined outer lateral face 201 matching with the corresponding one of the inclined faces 11 of the yoke 10, and the jaws 20 are placed in the space 3 with their faces 201 in slidable contact with the corresponding inclined faces 11 of the yoke 10. When the yoke 10 is moved upwardly in the figure relative to the jaws 20, the jaws are tightened to grip a specimen S, and when the yoke 10 is moved in the opposite direction, that is, downwardly in the figure, the jaws are loosened to release the specimen. The relative movement of the yoke 10 and the jaws is conducted by hand or a hydraulic cylinder.
The grips of the above-mentioned manual and hydraulic types have the following disadvantages.
In the manual type, since a handle is manually rotated to move the yoke relative to the jaws to grip or release a specimen, the operation is troublesome and it is difficult to have a sufficient gripping force.
In the hydraulic type, it is easy to operate the grip since mere handling of a selector valve suffices to supply or discharge pressure oil or air into or from the cylinder thereby to effect relative movement of the yoke and jaws of the grip. With pressurized air, however, it is impossible to obtain a sufficient gripping force. With pressure oil it is possible to obtain a large gripping force. To obtain a large gripping force, however, it is necessary to provide a source of pressure oil capable of producing a high pressure and a hydraulic cylinder of a large diameter resistive to high pressure, so that the whole device becomes bulky with a high manufacturing cost. In addition, since the pressure oil in the cylinder is blocked by means of a selector valve while the grip is holding a specimen, leakage of pessure oil will cause the gripping force to decrease.
Accordingly, the primary object of the invention is to provide a wedge type grip which is compact in size and capable of providing a large gripping force and has good operability.